dt-bindings: mtd: partitions: allow SafeLoader dynamic subpartitions

TP-Link SafeLoader partitioning means flash contains multiple partitions
defined in the on-flash table. Some of those partitions may have a
special meaning and may require describing additionally. Allow that.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221108093102.8360-1-zajec5@gmail.com